A RHIZOBIUM-MELILOTI MUTANT, LACKING A FUNCTIONAL GAMMA-AMINOBUTYRATE(GABA) BYPASS, IS DEFECTIVE IN GLUTAMATE CATABOLISM AND SYMBIOTIC NITROGEN-FIXATION

A Rhizobium meliloti mutant, CMF1 2:38, was isolated which was specifi cally defective in the degradation of glutamate as sole carbon and nit rogen source. Biochemical analysis of CMF1 2:38 revealed a reduction i n succinic semialdehyde dehydrogenase (SSDH) activity, the third enzym e of the gamma-aminobutyrate (GABA) bypass. Evidence is presented whic h suggests that the Tn5-induced mutation in CMF1 2:38 exists in a regu latory gene governing the expression of both NAD and NADP-linked SSDH activity. CMF1 2:38 nodulated alfalfa plants, but was reduced in its n itrogen fixation activity and biomass accumulating ability relative to the wild-type strain. The results presented in this study indicate th at the GABA bypass is a major mechanism of glutamate degradation in R. meliloti CMF1 and that glutamate catabolism via this pathway may play an important role in the symbiotic nitrogen fixation process.